1. Field of the Invention
The present invention relates to the manufacture of semiconductor devices, such as integrated circuits, transistors and diodes, wherein a local oxidation process is employed to locally oxidize a silicon substrate through the use of a mask made of a nitride-oxide double layer.
2. Description of the Prior Art
In the field of manufacturing bipolar integrated circuits, particularly bipolar memories, improvements have been made toward the increase of the density of the circuit elements to be included in an integrated circuit chip. Among techniques which have been employed is the oxide isolation technique which is suitable and advantageous for manufacturing bipolar integrated circuits which are to have a high speed or frequency of operation. This oxide isolation technique is described in the Peltzer, U.S. Pat. No. 3,648,125, issued Feb. 2, 1971.
In accordance with the oxide isolation technique, semiconductor isolation regions (diffused P-type regions), which isolate circuit elements from each other in conventional bipolar integrated circuits, are replaced by oxide regions which are formed by locally oxidizing portions of the epitaxial layers which are not masked by a silicon nitride layer.
In addition to the technique described in the above referred to patent, there are two variants of such an oxide isolation technique for forming bipolar integrated circuits, which are described in an article by W. D. Baker, et al., entitled "Oxide Isolation Brings High Density to Production Memories" published in Electronics, Mar. 29, 1973, pages 65-70. One of the variants described is the epitaxial-base process in which P-type epitaxial layers are employed as the base regions of bipolar transistors. The other variant is the double-diffused process in which the base regions are formed by impurity diffusion techniques.
Examples of semiconductor devices formed in accordance with such techniques are depicted in FIGS. 4 and 5 of the drawings of the present application which, respectively, illustrate an epitaxial base type oxide isolation integrated circuit and a double-diffused type oxide isolation integrated circuit.
In FIG. 4, upon a P-type substrate 1, in which an N.sup.+ region 2 has been diffused, an epitaxial layer 3 is formed. This epitaxial layer has a P-type base region, in which an N.sup.+ emitter region 11 is formed. Surrounding the base region and isolating the collector contact region 12, of N.sup.+ conductivity, is an oxide isolation region 7, which contains beaks 7a-7d at the edges thereof. Overlapping the emitter-base junction is a protective insulating film 9, and respective emitter, base and collector electrodes 13a-13c contact the respective surface portions of the device through the insulating and oxide layers.
The epitaxial-base process to form such a device has been principally employed to date, since it includes simpler manufacturing steps as compared with the double-diffused process (a decrease in the manufacturing steps reduces the number of design variables) and since the collector-to-emitter leakage current is small.
Although the double-diffused process is superior to the epitaxial-base process with respect to the resulting speed or frequency of operation of the completed device, the double-diffused process, which results in the formation of a device as depicted in FIG. 5, has serious drawbacks.
With reference to FIG. 5, in which like reference numerals to those employed in FIG. 4 are employed, the base region, rather than being epitaxially formed as shown in FIG. 4, is diffused to form a diffused base region 8. In order to avoid the formation of dislocations or defects in the silicon substrate beneath the nitride layer which is used to form the diffused regions, a nitride-oxide double layer film is employed for masking the silicon surfaces during the local oxidation of the silicon substrate. When the local oxidation of the silicon surface is carried out with the nitride-oxide double-layer film, silicon oxide beaks 7a-7d are formed due to the additional supply of oxygen in the lateral direction through the intermediate oxide layer. The formation of such "beaks" is described in an article by J. A. Appels, et al., entitled "Local Oxidation of New Technological Aspects" published in Volume 26, No. 3, of the Phillips' Research Reports, June 1971, pages 157-165.
In the double-diffused process, the oxide region 7, with the beaks, may be used as a diffusion mask forming the base region 8. The base diffusion results in the formation of a PN junction 38 which terminates at the surface of the silicon layer 3b, below the oxide beaks 7a, 7d. Where the impurity diffusion is followed by an etching process, the PN junction 38 may sometimes be left unprotected since the oxide beak is easily etched away. It is especially difficult to avoid leaving the PN junction unprotected when the base contact aperture is formed so as to be self-aligned with the edge of the oxide isolation region.
Furthermore, when the base electrode 13b is formed so as to contact a pair of the diffused base region 8 near the oxide isolation region 7 and extends onto the oxide beak 7a, the base electrode 13b may be short-circuited with the N type epitaxial layer (collector region) 3b, through a pin hole in the oxide beak.
In addition, a leakage channel may be formed at the interface of the base region 8 and the oxide beak 7 due to the small distance between the base electrode 13b and the collector region along the oxide isolation region 7.